1. Field of the Invention
This invention relates to a process for manufacturing a carbonaceous catalyst for the reduction of nitrogen oxides in exhaust gases, specifically for the catalytic reduction of nitrogen oxides in the presence of ammonia. Before use, the carbonaceous material is preferably treated with a halogen or a halogen compound to increase its catalytic activity.
2. Background Information
German Patent No. 23 50 498 discloses a process for the removal of nitrogen oxides from exhaust gases. In this process, an adsorption agent containing carbon, e.g. activated carbon, is first impregnated with an aqueous hydrogen halide solution, or with another compound which easily releases halogen. The adsorption agent is dried and then used by reacting the adsorption agent with the nitrogen oxide in the presence of amonia gas at temperatures between 60-150 degrees C. The catalytic activity of the carbonaceous material was found to increase as a result of the halogen treatment.
This process, however, has several disadvantages. First, during the reaction of the amonia with the nitrogen oxides, the halogen, e.g. bromine, is released from the catalyst, so that the catalytic activity of the catalyst decreases with use. Secondly, the released halogen reacts with ammonia to form ammonium halide. Thus, in order to remove the released halogen or the ammonium halide from the system so that they do not get released into the atmosphere, the first layer of adsorption agent must be used in conjunction with an additional layer of adsorption agent, in which the additional layer of adsorption agent is for adsorbing the released halogen or ammonium halide.
In addition, during the reduction of nitrogen oxides with ammonia on carbonaceous adsorption agents, ammonium nitrate and ammonium nitrite formation and enrichment, although negligible, have been observed when the reduction was carried out at temperatures around 100 degrees C. These substances, ammonium nitrate and ammonium nitrite, have a tendency toward spontaneous decomposition.